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368                           Impulsive Stress.

Stress  caused by  Impulsive  Load. — When a body
moves with a given velocity, its store of energy (or work capacity)

= —- ft. Ibs. (see p. 98).     If this be absorbed by an elastic

<>
material, we have:

work stored =    work given out

wv2         FIbs

-----     -. — x Aft (within elastic limit)

= total mean stresslbs x Aft (for all cases)

<j

and              Total mean stress -. wv*

in Ibs.              2ffAft

which is applicable to steam-hammers, pile-drivers, fly-presses, gun-
targets, &c.

If the fall of a weight deflect a beam, or stretch a crane chain
then—

work stored in weight ) __ J work done on material

«   .  _i n              > —. \    •           ,    . _,

in inch Ibs.

w

in inch Ibs.

JTibs

x A

and Flbs is the greatest total stress, or the steady load which would
produce the same strain A.

Stress caused by Heating and Cooling.—Experiment
shews that the expansion or contraction by heat or cold of a bar

•  f       4        i

:U

by

•'• '*/

of given material, is a regular quantity for each degree of tempera-
ture. When measured per inch length or breadth, and per degree
Fahrenheit, it is given in the following table : —